What point was Peter making to the people who crucified Jesus?
Peter was making the point that Jesus, the King the people had crucified and killed, was raised from the dead by God and could still be their King. David, being a prophet, had foreseen the resurrection of Christ, so the promise of an heir on David's throne pointed to Jesus.
Les explains that Peter, speaking to men and brethren, first notes that David is both dead and buried, with his sepulcher still present. Therefore David was not speaking of himself when he wrote of not being left in hell or seeing corruption. David, being a prophet, knew God had sworn with an oath that from the fruit of his loins He would raise up Christ to sit on his throne. Since a king sits on a throne, Peter's point is that the King they crucified and killed was raised by God from the dead and can still be their King. David, seeing this beforehand, spoke of the resurrection of Christ.